I don’t mind, but there are others who know more about Master Yin Shun’s ideas than I do. Nevertheless, I’ll give it a shot.
I understand Yin Shun to have suggested that the Saṃyukta Āgama/Saṃyutta Nikāya was the first collection of scriptures. The overall framework was that of the four noble truths and the three aṅgas. (For more details, see Bhante Sujato’s “A History of Mindfulness”.) As suttas were collected, they would be classified according to this scheme. Over time, however, this structure reached its use-by date and was changed to the structure we have now, that is, the four Āgamas/Nikāyas. During this process, suttas would have been reallocated to the new collections, so that the root Saṃyukta Āgama would have lost part of its content. At the same time, new suttas continued to accrue to each collection.
Because of this moving around and continuing accrual of suttas, it is not possible to put forward a general principle that the suttas of the Saṃyukta Āgama, as we have it now, are earlier than the suttas of the other Āgamas. Rather, we need to evaluate each sutta on its own terms to decide its antiquity.
Might it not be the case, nevertheless, that the suttas of the Saṃyukta Āgama are on average older than those of the other Āgamas? Perhaps, but it would be difficult to come to a definite conclusion. To me the right approach is to look at the suttas on an individual basis.